This invention relates to the field of control systems for production of liquefied gas (LG), and more specifically, to a process and system which controls LG production and LG temperature. It has particular but not exclusive application to liquefying natural gas (NG) to produce liquefied natural gas (LNG).
Systems for the liquefaction of natural gas (NG) by refrigeration in heat exchange means, especially using a multicomponent refrigerant, are in use throughout the world. Control of the LNG production process is important to operate a plant efficiently, especially when attempting to meet demands for incremental production for downstream processing or when attempting to adjust to external process disturbances. Essentially simultaneous and independent control of both the LNG production flow rate and temperature is important for LNG plant operation. By fixing and maintaining the LNG production rate, plant operators can adequately plan and achieve desired production levels as required by the product shipping schedule. Maintaining the temperature of the LNG within a specified range is important for downstream processing and the prevention of downstream equipment problems. Once regulatory control is achieved for the key variables, optimization strategies can be properly implemented. However, if regulatory control is not adequate, even standard day to day operation is adversely affected.
In typical NG liquefaction processes, natural gas is fed to the warm end of heat exchange means, having a liquefying section in which the natural gas is liquefied and a subcooling section in which the liquefied natural gas is subcooled, and the LNG outlet stream is withdrawn from the cold end of the heat exchange means. Some refrigeration duty in the liquefying section is provided by flashing a first refrigerant (“MRL”), provided by cooling in the heat exchange means the liquid portion of a phase separation of a multicomponent refrigerant (MR) and refrigeration duty in the subcooling section is provided by flashing a second refrigerant (“MRV”), provided by condensing in the heat exchange means the vapor portion of the MR phase separation. The remainder of the refrigeration duty in the liquefying section is provided by spent MRV from the liquefaction section. The refrigerants exiting the warm end of the heat exchanger means are combined, if not already mixed in the liquefaction section, compressed and precooled before return to the MR phase separation for recycle to the heat exchange means. A process having the aforementioned features is referred to herein as “a typical NG liquefaction process”.
U.S. Pat. No. 5,791,160 (Mandler et al; corresponding to EP-A-0893665) describes a natural gas liquefaction control scheme where LNG product flow rate and temperature are simultaneously and independently controlled by adjusting the amount of refrigeration. In the exemplified embodiments, the control variables (the ones having a set point that can be changed by the operator) of a typical NG liquefaction process include LNG product flow rate and temperature as well as the MRL/MRV ratio. Manipulated variables (the ones that are automatically controlled in response to operator setting of one or more of the control variables) include MR compressor speed and MR/LNG ratio. In this scheme the amount of refrigeration is adjusted after the actual LNG product flow rate has been changed in response to a change in the LNG product flow rate set point.
U.S. Pat. No. 6,725,688 (Elion et al; corresponding to WO-A-01/81845) describes a modification of Mandler et al with the object of maximizing power utilization. LNG product temperature and MRL/MRV ratio are retained as controlled variables and the manipulated variable is LNG/MRL ratio but LNG product flow rate cannot be independently set.
U.S. Patent Application Publication 2004/0255615 (Hupkes et al; corresponding to WO-A-2004/068049 & EP-A-1595101) describes the use of an advanced process controller based on model-predictive control to control a typical NG liquefaction process. The controller determines simultaneous control actions for a set of manipulated variables in order to optimize at least one of a set of parameters including the production of liquefied product whilst controlling at least one of a set of controlled variables. The set of manipulated variables includes MRL flow rate, MRV flow rate, MR composition, MR removal, MR compressor capacity and NG feed flow rate. The set of controlled variables includes the temperature difference at the warm end of the main heat exchanger, an adjustable relating to the LNG temperature, the composition of the refrigerant entering the MR phase separator, the pressure in the shell of the main heat exchanger, and the pressure and liquid level in MR phase separator.
There is a need to develop a simple and robust control scheme that allows control of LNG product temperature and flow rate without subjecting the heat exchange means to thermal stresses and without the need to manipulate the MR compressor and it is an object of the present invention to meet that need.